As is well known, the compressor case of a gas turbine engine powering aircraft is subjected to severe pressure and temperature loadings throughout the engine operating envelope and care must be taken to assure that the components remain concentric maintaining relatively close running clearances so as to avoid inadvertent rubs. Inasmuch as the engine case is thin relative to the rotor and stator components in the compressor section, it responds more rapidly to temperature changes than do other components. This is particularly true during periods of transient engine performance. Typical of these transients are throttle chops, throttle bursts, and the like. Obviously it is customary to provide sufficient clearances during these transients to assure that the rotating parts do not interfere with the stationary parts.
The problem becomes even more aggravated when the engine case is fabricated in two halves (split case) which is necessitated for certain maintenance and construction reasons. Typically, the halves are joined at flanges by a series of bolts and the flanges compared to the remaining portion of the circumference of the case is relatively thick and hence does not respond to thermal and pressure changes as quickly as the thinner portion of the case. The consequence of this type of construction is that the case has a tendency to grow eccentrically or out of round.
In certain instances in order to attain adequate roundness and concentricity to achieve desired clearance between the rotating and nonrotating parts, it was necessary to utilize a full hoop case for the highest stages of a multiple stage compressor. Since the stator components, i.e., stator vanes and outer air seals, are segmented the problem was to assure that the compressor maintained its surge margin notwithstanding the fact that the outer case would undergo large deflection at acceleration and deceleration modes of operation. The cavity that exists between the outer case and the inner case formed by the segmented stator components, being subjected to pressures occasioned by the flow of engine air through the various leakage paths, presented a unique problem. In the event of a surge, which is a non-designed condition, the pressure in the gas path would be reduced significantly. Because the air in the cavity is captured and cannot be immediately relieved, it would create an enormous pressure difference across the stator components, cause them to distort, with a consequential rubbing of the compressor blades, and a possible breakage.
In order to withstand this pressure loading and yet achieve the roundness and clearance control of the stationary and rotating components it was necessary to incorporate a mechanism that would tie the outer case to the segmented stator components. While it became important to assure that this rubbing did not occur, particularly where severe rubbing could permanently damage the blades and/or rotor/stator during surge, the mechanism that is utilized must be capable of withstanding this enormous load, yet be insensitive to fatigue. The most obvious solution to solving the load problem is to utilize sufficiently large bolts that could carry the load. The problem with this solution is that fatigue life is inversely proportional to the size of the bolt. The larger the diameter of the bolt the more sensitive it is to fatigue. The problem is more aggravated since the engine is designed to avoid surge and surge may be non-existing so the part used to solve the problem only has utility during a circumstance that may not occur. Thus, it is abundantly important that it doesn't present a maintenance problem, i.e. require early removal because of fatigue. Furthermore, it shouldn't be unduly heavy, since weight would impact overall engine performance.
I have found that I can obviate these problems by fabricating the fastener into two major component parts; 1) an outer spool threadably engaging the inner segmented stator vane/outer air seal assembly (case) and 2) a bolt threadably engaging an internal thread formed on the inner diameter of the outer spool that is supported to the outer full hoop case. The threads on the spool are dimensioned differently than the threads on the bolt so that the breakaway torque on the bolt is such that it will loosen before the spool loosens. The head of the bolt is dimensioned relative to the shank of the bolt such that it enhances fatigue life. By incorporating the spool into the fastener the design permitted the use of a bolt that had a longer shank than the heretofore known short shank designs, which improved the fatigue loading. It is contemplated that the end of the spool carries a washer like face that bears against the surface of the outer case so that the threads of the spool mating the threads of the inner case is insensitive to fatigue loading inasmuch as the spool is preloaded by this washerlike face. The spool is mounted so that it is always in compression which has the advantage of reducing fatigue of the bolt and stresses that occur as a result of a surge condition.